Probiotics: A Surprising Way to Beat Stress

Those trillions of microbes that live in your body—sorry!—have an astonishing influence on your state of mind and may affect your waistline

by Diane Lange

Photograph: Illustrated By Aad Goudappel

Have you ever been hit with gas pains while you were stressed out? Or battled butterflies in your stomach before doing something daunting? If so, you’ve experienced the chemical and electrical communication that goes on between the brain in your head and the one in your digestive tract, which is known as the small or second brain.

The brain in your gut is not a discrete organ like the one up top; it is a network of nerves within the layers of tissue lining the walls of your digestive tract. This network’s main job is to regulate the action taking place along the 30-foot tube extending from your esophagus to your anus and all the stops along the way: stomach, small intestine and large intestine. Long aware of the role these nerves play in the second-brain communication system, researchers have discovered a surprising new player—actually, about a trillion new players: the bacteria that live mainly on the inner walls of the large intestine.

These diverse bacteria perform several useful functions, such as digesting fiber, making vitamin K and some Bs and helping fend off the small amount of bacteria in your gut that could, in large enough numbers, make you sick. The “good” bacteria control the “bad” bacteria, either by producing substances that inhibit or kill the pathogens or by crowding out the troublesome bugs so it is difficult for them to adhere to the walls of the large intestine, says Kelly Scott Swanson, PhD, an associate professor in the department of animal sciences at the University of Illinois at Urbana-Champaign. Of the thousand or so kinds of bacteria in the gut, the most beneficial are probably the genera Lactobacillus and Bifidobacterium, notes Shekhar Challa, MD, a gastroenterologist in private practice in Topeka, Kansas, and author of Probiotics for Dummies. The term probiotics refers to good bacteria that live in food and certain supplements.

Researchers have known for years that stress can really rock this bacterial world. When your big brain senses a threat, glands pump out stress hormones that, among other things, trigger several chemical responses in your gut. As a result, the balance of good and bad bugs may be altered in favor of the -disease-causing bacteria, says Michaël Messaoudi, scientific and medical director of ETAP, a research laboratory in -Vandoeuvre-lès-Nancy, France. One study, for example, found that a hormone you produce when you’re stressed, norepinephrine, increases the virulence of Escherichia coli, a well-known pathogen. Another bad boy, Campylobacter jejuni, which causes the most common type of infectious diarrhea, may also surge when you’re under pressure.

The big news: In the past couple of years, scientists have discovered that just as the brain alters the gut bacteria, so too can these bacteria influence the brain. Not only do stress and your moods affect the functioning of your gut, but the bacteria in your large intestine may affect your mood and your emotional response to stress. For instance, animal experiments have shown that rises in Bifidobacteriaincrease levels of tryptophan circulating in the blood. Tryptophan is a precursor of the feel-good neurotransmitters serotonin and dopamine.

Probiotics as Prozac

If the bacteria in your gut can affect your mind, is it possible to manipulate these bugs to bring about desirable changes? A French team led by Messaoudi believes the answer is yes. The researchers gave healthy but moderately anxious people a daily probiotic supplement that contained Lactobacillus helveticusand Bifidobacterium longum. After 30 days, the volunteers who had taken probiotics showed decreases in anger and depression and an increased ability to solve problems. A later analysis showed similar mood-lifting effects in subjects who were less stressed.

Along the same lines, researchers in Ireland found that probiotics—in this case, daily sips of the good bacteria Lactobacillus rhamnosus—altered the behavior of mice placed in conditions that normally cause stress and depression. After a few weeks of the regimen, the mice were confronted with a maze, which normally creates high anxiety in a mouse. Yet the mice that had sipped the probiotic punch were as laid back as if they were on Valium, says John Cryan, PhD, chairman of the anatomy and neuroscience department at University College Cork, who directed these experiments. The treated mice also had lower levels of the stress hormone cortisol than their fellow creatures that didn’t get the Lactobacillus when stressed.

Cryan offers a theory for the calming effects of the Lactobacillus: The bacteria may secrete substances or interact with other substances in the gut that are taken in by the small brain and passed along the vagus nerve (which travels between the abdomen and the head) to the brain, where the chemical messengers activate receptors for GABA, a neurotransmitter that’s connected with relaxation. Another theory, he says, is that those substances act directly on the vagus nerve.

Building better bacteria

Does this mean you could overcome a case of nerves tomorrow by eating foods that contain good bacteria (such as yogurt) or downing probiotic supplements? The simple answer: We don’t know yet. “I can’t tell you to go to the health food store and get a certain probiotic pill to help your mood,” says Cryan, who notes that researchers are only beginning to figure out which strains of bacteria do what and to determine what doses are required to bring about desired effects.

Still, it makes sense to follow strategies that beef up the good bacteria in your gut: It might make you more stress resistant, probably will reduce inflammation in your digestive tract and will most likely lower your risk of developing type 2 diabetes.

The best starting point is what you eat. “Among the factors known to alter the profile of your bacterial world, diet is the strongest,” says Jane Foster, PhD, an associate professor in the department of psychiatry and behavioral neurosciences at McMaster University in Hamilton, Ontario. You can increase the amount of good bacteria in your body by consuming more foods that contain probiotics. Kefir, a fermented milk product, and cheese contribute probiotics, says Challa. Yogurt is a good source since, by the FDA definition, it contains Lactobacillus bulgaricus and Streptococcus thermophilus, and many manufacturers add other strains as well. Make sure the yogurt you buy has been refrigerated and the label confirms it contains “live” or “active” cultures (most supermarket brands are fine; frozen yogurts are iffier). Less obvious sources of probiotics include other fermented foods such as sauerkraut and kimchi (a Korean version of fermented cabbage) and naturally fermented pickles (fermented in briny water rather than vinegar; look for labels that don’t include vinegar).

You can also boost good bugs by eating more foods that contain prebiotics, substances that stimulate the growth and activity of desirable bacteria already living in your intestine. They help the good bacteria survive or prosper better than other bacteria, Challa says. Pre-biotics, such as the fiber in oatmeal, are soluble fiber, the type that dissolves in water. (Insoluble fiber, like that found in wheat bran, does not dissolve, which is why it contributes bulk to your stool.)

In a recent study, Swanson and his colleagues demonstrated that pre-biotics can improve the composition of your bacterial world. Subjects who ate snack bars containing 21 grams of one of two kinds of soluble fiber, either polydextrose or soluble corn fiber, increased their Faecalibacterium, a bacterium known to have anti-inflammatory properties. Soluble corn fiber also boosted Lactobacillus, which, among other things, increases the acidity of the large intestine, making it harder for disease-causing bugs to survive. “Prebiotic-rich foods include oats, chicory, bananas, wheat, garlic and onions,” says Challa. Other good sources of prebiotics are flaxseed, artichokes, barley, legumes and raw dandelion greens.

Although there’s no established guideline for the optimal amount of prebiotics, the usual nutritional advice is to eat 30 to 35 grams of fiber (soluble and insoluble) a day to improve your digestive functioning and reduce your risk of type 2 diabetes and other problems. Most Americans consume less than half that amount.

The supplement route

Odds are you will not eat enough yogurt or other probiotic- and prebiotic-rich foods each day to make a huge change in your gut bacteria. Taking daily supplements of probiotics or synbiotics (a combo of prebiotics and probiotics) may be enough to put you over the top. If you decide, in concert with your physician, to get supplements, opt for those that contain a variety of bacteria strains; two you definitely want are Lactobacillus and Bifidobacterium, advises Challa. And don’t drink anything hot for an hour after you take the pills, since heat destroys the bacteria.

There’s one situation in which doctors may recommend probiotics, and that’s when you take antibiotics, which typically decimate good bugs in your body along with the bad guys, potentially allowing more harmful ones to take over the colon. In this situation, “some people get diarrhea,” says Braden Kuo, MD, instructor of medicine at the Harvard Medical School and director of the GI Motility Laboratory at Massachusetts General Hospital, both in Boston. If you really do need antibiotics, taking probiotic supplements may restore the healthy bacteria population in the gut. Your goal is to have a gastrointestinal tract dominated by healthy bacteria that can improve your mental and physical functioning while also fending off the dangerous bacteria.